1. Field of the Invention
The present invention relates to a method for achieving alignment between objects, and more particularly relates to a method for automatically achieving alignment in a computer aided positioning and installation system with respect to at least two objects to be overlapping.
Generally, in various kinds of installation systems, it is important to achieve alignment between objects to be overlapped. For example, in a system for manufacturing printed circuit units, it is important to achieve alignment between first objects, that is a plurality of terminal pads mounted on the surface of a printed board, and second objects, that is a plurality of leads from an IC (Integrated Circuit) package. Each of the leads is overlapped with a respective terminal pad, so that the leads and the respective terminal pads are correctly joined with solder. Since the IC package has many leads, for example several tens of leads, it is very difficult to achieve alignment between so many leads and the respective terminal pads with a high degree of accuracy and in a very short time.
2. The Prior Art
In the prior art, many kinds of methods for achieving alignment between leads and respective terminal pads have been proposed and put into practical use. A first method is carried out by utilizing a beam-splitter and a microscope. In the first method, the leads of an IC package are located about 30 cm above the terminal pads and the IC package is movably supported, in a horizontal plane, by a mechanical hand. An inspector observes the terminal pads directly through the microscope and the beam-splitter. At the same time, he observes an image of the leads through the microscope, which image is reflected from the beam-splitter. Then, he controls the mechanical hand and shifts the location of the IC package so as to coincide the image of the leads with the respective terminal pads. When coincidence is obtained, he shifts the package downward, so that the leads correctly contact the respective terminal pads. However, this first method has a defect in that it is difficult to achieve alignment with a high degree of accuracy and in a very short time.
A second method is carried out by utilizing a magnifying apparatus. In the second method, the image of the leads and also the image of the terminal pads are projected on a screen. The inspector controls the mechanical hand so as to coincide both images with each other on the screen. When coincidence is obtained, he shifts the package downward, and then the leads correctly contact the respective terminal pads. However, this second method has the same defect as that of the first method.
A third method is carried out by utilizing an optical detector. In the third method, the optical detector detects the location of the end lead being projected from the corner of the IC package, and the inspector controls the mechanical hand so as to move the end lead to a position located right above the corresponding terminal pad. However, this third method has a defect in that it is difficult to achieve highly accurate alignment between the remaining leads and the remaining terminal pads. This is because, although the end lead, which acts as a scale for alignment, must be projected straight along a line from the IC package, this end lead is often bent and forms a bow due to the mechanical impact during, for example the transport of the IC package.
A last method is the so-called pattern-matching method. In the last method, the destination of the position of each lead is stored, in advance, in a computer storage device. The computer controls the position of each lead so as to coincide the actual present position of each lead with the stored destination of the position of the lead. However, this last method has defects in that, firstly, it is difficult to achieve highly accurate alignment therebetween, because, even though coincidence would have been obtained, the positions of the terminal pads often slightly deviate from the predetermined positions due to a small flexure developed on the printed board because of the variation of atmospheric temperature. Secondly, it is impossible to carry out this last method if the destination of the position of each lead is not prepared and stored, in advance, in the storage device of the computer. This is very time consuming.